This invention relates generally to optical communication devices and arrangements, and, in particular, to a push-pull thermooptic switch which is driven from a single control signal.
Wavelength Division Multiplexing (WDM) control devices, such as wavelength add-drops (WADs), wavelength selective cross connects (WSCs), and dynamic gain equalization filters (DGEFs), often consist of a demultiplexer and a multiplexer connected by an array of switches. A low-loss, compact, mass-produceable way to make the switches is to use a planar arrangement of thermooptic Mach-Zehnder (M-Z) interferometer switches in silica waveguides.
As discussed in my above-identified patent application, a push-pull thermooptic Mach-Zehnder interferometer switch has several advantages over conventional thermooptic Mach-Zehnder switches, such as reduced power consumption and polarization dependence and constant power dissipation. With reference to FIG. 1, there is shown a push-pull thermooptic Mach-Zehnder interferometer switch as described my above-identified patent application. As shown, each waveguide arm 101 and 102 includes a thermooptic phase shifter. A thermooptic phase shifter is simply a heater, (e.g., 110 and 112) deposited over the waveguide arm (101 and 102, respectively) that causes the refractive index of the waveguide arm material to change via a temperature change when electrical signal (CONTROL 1, CONTROL 2, respectively) is applied to the heater. Usually the two path lengths (i.e., the lengths of arms 101 and 102) between the input coupler 120 and the output coupler 130 are designed to be equal when the thermo-optic phase shifter is undriven, although sometimes there is about a quarter-wavelength element 140 to provide a phase bias in the switch.
Notwithstanding the improvements of my previous push-pull thermooptic Mach-Zehnder interferometer switch, that design required a separate control lead (111, 112) and drive signal [CONTROL 1, CONTROL 2, respectively] for each arm. Since WDM control devices, often consists of a demultiplexer and a multiplexer connected by an array of such push-pull thermooptic interferometer switches, it would be desirable to reduce the number of control leads and drive signals needed to operate each push-pull thermooptic interferometer switch.
In accordance with the present invention, I have recognized that a push-pull thermooptic interferometer switch can be controlled using only one control lead and one drive signal per switch. Advantageously, using only control signal lead and one electrical driver per switch can greatly reduce the complexity of the electronics needed to drive a switch array. My single control lead push-pull thermooptic interferometer switch can be driven by controllable voltage or current signals. The connections of the control and other signal leads to an array of these switched can be made in a planar manner without the need for crossover paths.
More particularly, my invention is a push-pull driven two-arm thermooptic interferometer switch comprising
a first two-terminal phase control element to control the optical phase in a first arm, a first terminal of the first phase control element being connected to a fixed electrical signal,
a second two-terminal phase control element to control the optical phase in a second arm, where a second terminal of the second phase control element is connected to the second terminal of the first phase control element, and
a single controllable electrical signal applied across the first and second terminals of the second phase control element, wherein the level of the controllable electrical signal controls the optical phase in a first and second arms by controlling the current flow in the first and second arms, respectively.
In one embodiment, the single controllable electrical signal includes a controllable voltage source applied to the second terminal of the second phase control element and a fixed voltage source applied to the first terminal of the second phase control element. In a second embodiment, the controllable electrical signal includes a fixed current source applied to the second terminal of the second phase control element and a controllable current source applied to the first terminal of the second phase control element.